1. Field of the Invention
The present invention relates to a poly-ethylene resin composition having an excellent balance of strength, rigidity, and gloss and excellent in moldability (or shapability), heat sealability of through contaminant and low temperature resistance properties. More particularly it relates to a polyethylene resin composition and film thereof, suitable for use as heavy duty packaging bags such as for organic fertilizer.
2. Description of the Related Art
At the present time, industrial heavy duty bags comprised of polyolefin resin film are being used for many packaging bags such as for organic fertilizer since they have superior moisture-resistance and can be easily sealed air-tightly by heat sealing the openings. Such polyolefin resin film heavy duty packaging bags are produced by shaping high pressure low density poly-ethylene resin manufactured by the high pressure method, or high pressure low density polyethylene resin copolymerized with about 3 to 6 mole% of vinyl acetate, to a thickness of about 200 .mu.m. These high pressure low density polyethylene films are inferior in strength, rigidity, and gloss compared with linear low density polyethylene resin film manufactured by the medium and low pressure methods.
On the other hand, in recent years, to conserve on resources, various types of packaging materials have been made thinner. Due to the above-mentioned characteristics, the use of linear low density polyethylene resin with large strength and rigidity is being increased. However, linear low density polyethylene resin places a large load on the motor of the extruder due to its moldability, in particular in inflation molding, and the bubble stability is poorer than the case of high pressure low density polyethylene resin. In the manufacture of a thick film, due in part to the insufficient cooling of the bubbles, the bubbles become unstable and formation of film becomes difficult. Further, the film surface easily becomes rough. Therefore, film-forming machines using linear low density polyethylene resin as materials are given larger lip clearance of the dies and make use of screws with small compression ratios, motors with large capacities, and devices for strengthening the cooling. It is difficult to form a film by a machine for high pressure low density polyethylene with a small lip clearance and small motor capacity.
Therefore, in consideration of the moldability, the method has been used of mixing high pressure low density polyethylene resin into the linear low density polyethylene resin material, but in this case, the rigidity and strength decline. Further, to alleviate the above-mentioned problems in the use of linear low density polyethylene resin as the sole material, it is necessary to broaden the distribution of molecular weight and increase the molecular weight. A film obtained in this way can satisfy requirements of rigidity, but is inferior in strength and gloss.